Ceramic sealing structure



United States Patent O 3,295,005 CERAMIC SEALING STRUCTURE Allan L. Poellet and Virgil J. Barczak, Royal Oak, Mich., assignors to Champion Spark .Plug Company, Toledo, Ohio, a corporation of Delaware Filed Oct. 28, 1963, Ser. No. 319,354 6 Claims.- (Cl. 313-137) The present invention relates to electrical apparatus of the type having a ceramic body sealed Within a metal housing; and more particularly to electrical discharge devices such as igniters, spark plugs, and the like.

Electrical discharge devices comprise a metal electrode that is centrally located and spaced from a tubular metal housing by means of an annular ceramic insulating member. Inasmuch as the electrical discharge devices with which we are concerned extend into high pressure combustion chambers, a suitable seal must be provided between the center electrode and the insulating member and between the insulator and the surrounding housing. The problem of producing a suitable seal between the various members is complicated by the fact that the seal must withstand hot burning gases, and the coefficient of expansion of the ceramic insulator may differ considerably from that of the metal parts.

A method for uniting a ceramic insulator member, a center electrode and a metal shell and forming a suitable seal is disclosed in British Patent 397,381 and in United States Patent 3,013,174. The ceramic insulator member is made in two parts that are positioned in end to end relationship between the center electrode and housing. A powdered glass is positioned between the two insulator portions, and the structure is heated until the glass becomes molten. Thereafter the two insulator parts are pressed together to cause the molten glass to run between the insulator and center electrode and between the insulator and the outer housing.

A problem has been encountered with this type of construction in that cracks occur in the ceramic insulators of a large percentage of the electrical discharge devices of this construction which are produced.

Accordingly, an object of the present invention is the provision of a new and improved construction of electrical discharge device wherein a molten glass seal material is compressed between insulator sections to effect a seal between the insulator sections and the center electrode and housing without causing cracks in the insulator sections.

The invention resides in certain constructions, and combinations, and arrangements of parts, and further objects and advantages will become apparent to those skilled in the art to which the invention relates from the following description of a preferred embodiment described with reference to the accompanying drawing forming a part of this specification, and in which:

The solitary figure of the drawing is a fragmentary sectional view, greatly enlarged, through the seal portion of a jet engine igniter.

In retrospect, it is believed that cracks occur in the ceramic insulator sections of the above described structures by reason of the fact that the molten glass between the metal shell and the insulator sections solidifies before the molten glass between the center electrode and the insulator section solidifies. Shrinkage of the metal shell during the cooling process, following solidification of the adjacent seal material, exerts a hoop stress through the solidified seal material to the insulator sections. Inasmuch as glass within the insulator sections solidifies at a later time than does the sealing material on the outside of the insulator section, the inwardly directed force upon the ends of the insulator sections is generally un- Patented Dec. 27, 1966 opposed and the ends of the insulator sections are placed under shear.

According to the invention, the compressive forces upon the ends of the insulator sections is greatly reduced by placing a comparatively soft solid material, which melts at a temperature above that .of the sealing material, between the metal shell and the insulator sections to reduce the compressive forces on the ends of the insulator sections during the cooling of the assembly following the melting of the seal material.

While the invention may be otherwise embodied, it is herein shown and described as embodied in an electrical discharge device, or igniter, of the type which is used to supply starting spark in a jet engine. It will be appreciated, however, that the invention may be used to advantage in a large number of different applications wherein high temperature gas impermeable ceramic seals must be made between tubular outer metal shells and ceramic structures located within the shells.

The igniter shown in the drawing comprises a generally tubular outer metal shell 10 having an annular inwardly extending flange 11 on its lower end which forms a ground electrode. The other electrode of the igniter is formed by a longitudinally extending metal rod 12 which is centrally located within the tubular shell 10 and which extends through and beyond the annular inwardly extending flange 11 to provide a spark gap of approximately 0.100 inch from the center electrode 12 to the flange 11. The center electrode 12 is insulated from the shell 10 by a tubular ceramic insulator 13 that is made in two parts which are spaced axially from each other. The ceramic insulator 13 includes an inner section 14 and an outer section 15, and the adjacent ends of the inner and outer portions are reduced in cross section as at 16 and 17 respectively to provide predetermined annular clearance with the sidewalls of the housing 10. Ceramic sleeves 18 and 19, which will later be described in detail, are positioned in the predetermined annular clearance provided by the areas of reduced cross section 16 and 17, and a ceramic sealing material 20 extends between the center electrode 12 and housing 10 in the area between the inner and outer ceramic insulator portions 14 and 15.

The structure shown in the drawing is assembled by placing the ceramic sleeve 18 over the upper end of the inner ceramic insulator section 14 and inserting this assembly into the shell 10. The center electrode 12 is then inserted through the insulator section 14 until the enlarged portion 21 of the electrode abuts the end of the insulator section 14. A predetermined amount of a glass powder is then placed on top of the insulator section 14 and the sleeve 18. The ceramic sleeve 19 is then placed around the reduced cross section portion 17 of the insulator section 15, and the insulator section 15 with the sleeve 19 thereon is slipped over the center electrode 12 and brought down upon the powdered glass. The assembled parts are then heated to melt the glass, and when the glass is in a molten condition, the insulator section 15 is forced down upon the glass. The molten glass is thereby caused to flow between the ceramic sleeve 18 and 19 and the shell 10, as well as between the sleeves 18 and 19 and the insulator sections 14 and 15 respectively. The molten glass also flows between the center electrode 12 and insulator section 15 but is prevented from flowing between the center electrode 12 and insulator section 14 by the enlarged portion 21 of the electrode 12.

After the glass has been melted and the molten glass has been forced between the various parts to effect good sealing contact, the assembly is allowed to cool down. Inasmuch as the shell 10 is made from a conventional metal, it contracts more than does the inner ceramic parts, during the cooling of the assembly.

The glass material 20 solidifies first adjacent the shell and the solidification proceeds slowly toward the center electrode. At one period during the solidification of the glass, the glass between the center electrode 12 and the ceramic insulator sections is molten while the remainder of the glass is solidified. Cooling down of the shell 10 during this period applies compressive forces upon the ceramic sleeves 18 and 19 and the reduced cross section portions 16 and 17 of the insulators 14 and 15 respectively.

According to the invention, the ceramic sleeves 18 and 19 are made from a material which is deformed by the contraction of the shell 10 at a compressive load that is below the breaking strength of the reduced cross section portions 16 and 17 of the insulators 14 and 15 respectively. Inasmuch as the glass within the insulator sections 14 and 15, if any, is molten at the period with which we are concerned, the compressive force which is transmitted through the ceramic sleeves 18 and 19 must be resisted solely by the strength of the portions 16 and 17 of the insulators 14 and 15 respectively. The amount of compression which the sleeves 18 and 19 must accom modate will depend upon the metal from which the shell is made, and the maximum force which can be allowed to be transmitted through the sleeves 18 and 19 will de pend upon the size and strength of material from which the insulators 14 and 15 are made. The sleeves 18 and 19, therefore, can be made from various materials having various strengths so long as the sleeves can be re duced in volume by the necessary amount to accommodate the shrinkage of the shell 10 at a compressive load that is less than the radially inwardly directed force which produces cracking of the insulator sections 14 and 15.

In one embodiment of spark plug which has been made, the ceramic insulators have been made of a fully fired or vitreous alumina having a compressive strength of approximately 350,000 p.s.i. The shell 10 was made of an A.I.S.I. type 446 ferritic stainless steel, and the sleeves 18 and 19 were made of a powdered material comprising 99% alumina and 1% bentonite, which was compacted at 10,000 p.s.i. and calcined at 2050 F. The calcined material prepared in this manner crushes under a compressive load of approximately 500 p.s.i. What is more, the material has a porosity of 37.8% so that its volume can be reduced by an appreciable amount under a load that is less than the compressive load which will produce cracking of the insulator sections 14 and 15.

It will be understood that powders of practically any ceramic can be loosely compacted to provide a porosity comparable to that of the above described alumina bentonite material. Likewise the strength of sintered powdered materials depends upon the time and temperature at which the material is sintered, and by sintering at the proper temperature, any required strength of sleeve can be provided. Examples of other materials from which the sleeves 18 and 19 can be made will include magnesium oxide, mullite, various silicates, various aluminates, spinels, titanium oxide, zirconium oxide, etc., so long as they are compacted and sintered at the proper pressures and temperatures.

It will be apparent that the objects heretofore enumerated as well as others have been accomplished, and that there has been provided a fused ceramic seal oonstruction for effecting a seal between an outer metal housing having a high coetficient of expansion and an internal ceramic structure having a lower coefficient of expansion.

While the invention has been described in considerable detail, we do not wish to be limited to the particular embodiment shown and described, and it is our intention to cover hereby all novel adaptations, modifications, and

arrangements thereof which come within the practice of those skilled in the art to which the invention relates.

What we claim is:

1. In a spark plug and the like, an outer shell of generally annular cross section, a ceramic insulator in said shell having a portion of generally annular cross section spaced apart from said shell, a sleeve positioned in said space between said insulator and said shell, and a sealing material extending between said sleeve aid said shell and bonded to said shell, said sleeve consisting of a compressible material which melts at a temperature above the melting temperature of said sealing material, said sleeve having been reduced in volume by the radially inwardly directed compressive force incident to the cooling of said shell from a temperature at least as high as the melting temperature of said sealing material to ambient conditions, and said sleeve being sufficiently compressible that the internal compressive forces produced in the sleeve by said volume reduction is less than that required to transmit sufiicient force to said insulator to crack said insulator.

2. A method of producing a gas impervious seal between a metal shell and an internal ceramic insulator spaced from said shell comprising: providing a solid compressible material of suflicient size, shape, and compressibility to accommodate the reduction in volume between said shell and insulator incident to the cooling of said shell and insulator from a predetermined temperature to ambient conditions while producing internal compressive forces that are less than that required to crack the insulator, placing said solid compressible material between said insulator and said shell, placing a vitreous material which melts at a temperature generally below said predetermined temperature in said shell in such position that said vitreous material runs between said shell and compressive material and said compressible material and said insulator when melted, heating said shell, vitreous material, and compressive material to said predetermined temperature, and allowing the resulting assembly to cool sufirciently for said vitreous material to solidify, whereby said compressible material is deformed upon the cooling down of said shell without transmitting sufficient force onto said insulator to crack said insulator.

3. The structure of claim 1 wherein said shell is a ferritic stainless steel, said insulator is of a hard vitreous alumina, and said sleeve crushes under a compressive load of approximately 500 pounds per square inch, and has a porosity of approximately 37.8%.

4. The structure of claim 1 wherein said sealing material is a vitreous material.

5. A spark plug and the like comprising: a metal shell of generally annular cross section, a metal electrode dis- .posed within said shell and extending longitudinally thereof, a pair of ceramic insulator members of generally annular cross section telescoped over the metal electrode and spacing said electrode from said shell, the adjacent ends of said ceramic insulator members having predetermined clearance with respect to said metal shell, a pair of sleeves respective ones of which are positioned in said clearance between respective ones of said insulator members and said shell, and a sealing material intermediate said ceramic insulator members filling the area between said shell and electrode and bonded to each, said sealing material extending between said shell and at least a portion of said sleeves, said sleeves consisting of solid compressible materials which melt at a temperature above the melting temperature of said sealing material, said sleeves having been reduced in volume by the radially inwardly directed compressive force incident to the cooling of said shell from a temperature at least as high as the melting temperature of said sealing material to ambient conditions, and said sleeves being sufficiently compressible that the internal compressive forces produced therein by said volume reduction is less than that re- 5 6 quired to transmit suflicient force to said insulators to References Cited by the Examiner crack said insulators. UNITED STATES PATENTS 6. The structure of claim 5 wherein said shell is of a Y ferritic stainless steel, said insulator mern'bers are of hard 2,959,703 11/1960 Hastmgs 313-445 alumina, and said sleeves crush under a compressive load of approximately 500 .p.s.i. and have a porosity of ap- 5 JOHN HUCKERT Pnmary Examiner proximately 37.8%. R. F. POLISSACK, Acting Examiner. 

1. IN A SPARK PLUG AND THE LIKE, AN OUTER SHALL OF GENERALLY ANNULAR CROSS SECTION, A CERAMIC INSULATOR IN SAID SHELL HAVING A PORTION OF GENERALLY ANNULAR CROSS SECTION SPACED APART FROM SAID SHELL, A SLEEVE POSITIONED IN SAID SPACE BETWEEN SAID INSULATOR AND SAID SHELL, AND A SEALING MATERIAL EXTENDING BETWEEN SAID SLEEVE AID SAID SHELL AND BONDED TO SAID SHELL, SAID SLEEVE CONSISTING OF A COMPRESSIBLE MATERIAL WHICH MELTS AT A TEMPERATURE ABOVE THE MELTING TEMPERATURE OF SAID SEALING MATERIAL, SAID SLEEVE HAVING BEEN REDUCED IN VOLUME BY THE RADIALLY INWARDLY DIRECTED COMPRESSIVE FORCE INCIDENT TO THE COOLING OF SAID SHELL FROM A TEMPERATURE AT LEAST AS HIGH AS THE MELTING TEMPERATURE OF SAID SEALING MATERIAL TO AMBIENT CONDITIONS, AND SAID SLEEVE BEING SUFFICIENTLY COMPRESSIBLE THAT THE INTERNAL COMPRESSIVE FORCES PRODUCED IN THE SLEEVE BY SAID VOLUME REDUCTION IS LESS THAN THAT REQUIRED TO TRANSMIT SUFFICIENT FORCE TO SAID INSULATOR TO CRACK SAID INSULATOR. 